Method and apparatus for growing large monocrystals of nonmetallics



May 13, 1969 E. F. NICHOLS ET AL 3,443,891

METHOD AND APPARATUS FOR GROWING LARGE MONOCRYSTALS OF NONMETALLICS Sheet 1 of 4 Filed June 6, 1966 FIG. I

INVENTORS E.F. Nichols K.H. lvey H.R. Shell ii-WM ATTORNEYS May 13, 1969 E. F. NICHOLS ET AL 3,443,891

METHOD AND APPARATUS FOR GROWING LARGE MONOCRYSTALS OF NONMETALLICS Filed June 6, 1966 Sheet 2 of 4 FIG. 2.

I/VVENTORS E.F. Nichols K.H. lvey H.R. Shell 2, I c WWW ATTORNEYS May 13, 1969 E. F. NICHOLS ET AL 3,443,891

METHOD AND APPARATUS FOR GROWING LARGE MONOCRYSTALS OF NONMETALLICS Filed June a, 1966 Sheet 3 094 "WE/Woks E. F. Nichols K. H. lvey H.R. Shell FIG. 30.

ATTORNEYS May 13, 1969 E. F. NICHOLS ET AL 3,443,891

METHOD AND APPARATUS FOR GROWING LARGE MONOCRYSTALS OF NONMETALLICS Filed June 6, 1966 Sheet 4 of 4 FIG. 4

E.F. Nichols K.H. lvey H.R. Shell ATTORNEYS 3,443,891 METHOD AND APPARATUS FOR GROWING LARGE MONOCRYSTALS OF NONMETALLICS Earl F. Nichols, Andersonville, Tenn., and Kenneth H. Ivey, Adelphi, and Haskiel R. Shell, Langley Park, Md., assignors to the United States of America as represented by the Secretary of the Interior Filed June 6, 1966, Ser. No. 556,807 Int. Cl. C01b 33/24 US. Cl. 23-110 8 Claims ABSTRACT OF THE DISCLOSURE An arc furnace melts a small furnace charge of micaproducing material, after which the furnace electrodes are immersed in the melt. As more mica-producing material is charged to the furnace on top of the melt, the electrodes are slowly, continuously raised through the charge to melt the same and establish a thermal gradient along which mica crystals will grow. A cover of unmelted charge is continuously maintained on top of melted charge to prevent the loss of volatile constituents.

This invention relates to a process for growing large monocrystals of mineral substances and inorganic compounds and an improved apparatus for performing the procedure.

Large monocrystals of silicate-containing and oxidecontaining mineral substances and inorganic compounds are very useful in industry and have been the object of much research. Crystallization from a solution is a well known technique for producing such crystals, but the production rate of this process is low, and complex equipment may be necessary. Moreover, each solution system must be worked out separately because of solvent reactions, and for many desirable crystalline substances suitable solvents have not been found or do not exist.

Many techniques for crystallization from a melt have been developed for the production of silicateand oxidecontaining substances as discussed in Crystal Growth by H. E. Buckley, published by John Wiley & Sons, 1962. Exemplary of this general method is the flame fusion process, wherein powder is fed into a very hot flame, melted, then immediately deposited on a solid. This process is basically very simple, but suffers from the fact that the crystals so produced are heavily strained and have many imperfections. In addition this method is limited to fairly simple feed batches of low volatility which will not vaporize before crystallization begins.

One of the more successful melt techniques involves anisotropically growing crystals in a thermal gradient. In this process a batch of feed material is melted in an electrode furnace such as an internal resistance or are resistance furnace, and the melt subsequently allowed to cool whereby crystals begin to grow in the coolest portion of the melt (when this portion cools to a certain temperature), and each crystal continues to grow in the direction of the hottest portion of the melt. However, the internal resistance furnace, as shown, for example, in US. Patent No. 2,711,435, cannot be used for many substances and does not yield large monocrystals, although it exercises some control over the thermal gradient. Likewise, the arc resistance furnace has been unsatisfactory in that it has not produced high quality monocrystals. In particular, it has not been successful at all for the synthesis of crystals of substances containing volatiles (such as the fluoromicas).

An object of this invention is to provide an improved process for producing monocrystals along a thermal gradient in a resistance type furnace.

nited States Patent ice A further object is to produce large quantities of high quality monocrystals by inducing crystal growth mainly in one general direction.

A further object is to maintain relative motement between the furnace charge and a pair of electrodes.

A further object is to prevent volatile constituents in the charge from escaping by maintaining solid charge around melt formed during the process whereby volatilecontaining substances such as the fluoromicas can be synthesized.

A further object is to provide an apparatus for removing gases that may escape from the furnace at the start of the operation while at the same time providing for ready access to the furnace by a pair of electrodes.

For a more detailed understanding and for further objects and advantages of the invention reference is to be had to the accompanying drawings in which:

FIG. 1 is a fragmentary view, partly in section, of the furnace and the electrode moving means;

FIG. 2 is a fragmentary, sectional view of the furnace, illustrating the manipulation of the charge and electrodes;

FIG. 3 is an elevational, sectional view of the furnace and hood structure;

FIG. 3a is a perspective view from below of the furnace hood; and

FIG. 4 is a top view of the furnace and hood structure shown in FIG. 3.

Referring to FIG. 1, an electric furnace 1, used to perform the process of the present invention, is shown comprising a generally vertical casing providing a receptacle for a charge of materials. First, a layer 2 of raw material consisting of, for example, mica flakes, is placed or formed in the bottom of furnace 1 to act as electrical insulation and to prevent the subsequently added feed charge from being contaminated by substances in the furnace walls during the high temperature operation. On top of this layer is placed a small amount of the charge 3. When it is desired to produce a synthetic fluoromica, the charge might consist of a mixture of silica, alumina, magnesia, potassium silicofiuoride (K SiF and potash feldspar. A pair of electrodes 4 made of carbon or graphite are then lowered into the furnace from the top thereof to form an arc about 1" above the charge. Movement of the electrodes through electrode holder means 4a is accomplished by, for example, pull chains 5 attached to porcelain insulators 6 connected to the top of the electrodes. In turn the pull chains are driven by a shaft 7 and sprockets 7a, the shaft being driven by chain drive means 8 connected to motor 9 through a gear reduction unit 10.

When enough melt 11 has formed to quench the arc, the electrodes are immersed therein, forming an arc resistance type of power input so that heat may be rapidly transmitted to surrounding unmelted charge. Remaining feed charge is then added to the furnace by way of, for example, a feed chute 12, shown in FIG. 2, and the electrodes are continuously and slowly raised in a substantially straight path through the added charge to progressively fuse more of the charge from the bottom portion to the top. The addition of remaining feed charge and the electrode raising operation can be performed simultaneously. Vertical electrode movement through a straight path establishes a thermal gradient between the bottom cooler section of the melted charge and the warmer section of the melted charge surrounding the ends of the electrodes, whereby crystals form in the bottom section and grow in the direction of the warmer section. The electrodes are raised at a rate correlated with the optimum rate of crystal growth in the melted material. Furthermore, electrode movement along with power input and the rate of adding feed material to the furnace are adjusted to keep the formation of crystal nuclei at a minimum in the warmer sections of the melt above the bottom, coolest section of the melted charge. Minimum nuclei formation in the warmer sections of the thermal gradient results in substantially parallel, overlapping large monocrystals extending in the direction of the thermal gradient, whereas the formation of too many nuclei in the warmer sections results in small, randomly oriented crystals throughout the charge.

Relative movement between the electrodes and th charge need not be effected by moving the electrodes. The desired thermal gradient could also be established by holding the electrodes fixed and moving the furnace by suitable means.

Referring to FIGS. 3 and 4, a cross-section and top view are respectively shown therein of a furnace that can be used to carry out the process of this invention. Tapered walls 13 slide over a base 14 to comprise the main body of the furnace. The shape of the walls allows them to be readily removed by a crane. Perforated water sprinkler ring 15 facilitates cooling of the furnace. A splash shield 16 attached to the furnace by, for example, ribs 16a, prevents sprinkling water from entering the furnace interior. Cooling water running down the side of the furnace walls falls into catch basin 17 wherein the water cools the bottom-most section of the furnace since this section is spaced from the catch basin by spacers 18. A hood 19 provides for the removal of fumes that may occur at the start of the operation. The hood as shown in FIGS. 3, 3a and 4, consists of a ring-shaped chamber 20 which overlies, in close proximity, the peripheral area of the furnace opening and the perimeter 13a defining the opening. A duct 21 projecting outward from the chamber 20 is directly connected to a suitable exhaust fan (not shown), and thereby provides for the ready removal of furnace fumes that pass into chamber 20 through the continuous bottom opening 22 opposite the peripheral area of the furnace opening and furnace perimeter 13a. The configuration and location of hood 19 enables a plurality of electrodes to be easily raised or lowered through the furnace opening without contacting any parts of the hood. At the same time, the hood provides for efficient removal of fumes that may arise from the furnace, although it overlies only a minor portion of the furnace opening. Electrodes 3, which project through the opening of the furnace need not be of the arc type. For example, a resistor 40 may connect the ends of the electrodes so that the melt is initially started by internal resistance rather than by an arc.

The following examples illustrate ways in which the principle of the invention has been applied, but are not to be construed as limiting its scope.

EXAMPLE 1 Feed materials were mixed in the following proportions:

Weight percent K SiF 19.65 Potash feldspar 25.87 MgO 29.76 A1 7.41 SiO 19.11

Raw batch material pounds 8,800 Maximum power input kw 192 Average power input kw 164 4 Total power input kw. h-.. 1,310 Running time hours 8 Crystalline product yield pounds 4,050 Yield per kw. h. do 3.1 Melt height inches 56 Melt diameter do 38 Diameter of electrodes do 4 Rate of electrode movement inches/hour 8 Total man hours used hours 30 Over 4,000 pounds of synthetic fiuorphlogopite were made in 8 hours. The mica crystals were highly oriented in the same direction, showing that a strong thermal gradient was obtained. Though of good quality, the crystal size was small indicating that the rate of electrode rise was too fast thereby allowing too many crystal nuclei to form in the liquid melt in the warmer sections of the thermal gradient above the bottom, coolest section of the melted charge. However, this example demonstrates the feasibiliity of the arc resistance-continuous electrode movement system for synthesis even when volatile constituents are present.

EXAMPLE 2 A composition similar to that of Example 1 was melted in essentially the same manner as employed in Example 1 except that the continuous rate :of electrode rise was 4.50 inches per hour, and the experiment was run for 15 /2 hours. 5560 pounds of crystalline fluorphlogopite was produced.

LIELT SCHEDULE Volts Amps. Kw

40 2000 Start MELT STATISTICS Raw batch material -pounds 6000 Maximum power input kw 151 Average power input kw 123 Total power input kw. h 1982 Running time hours.. 15% Yield of crystalline product ..pounds 5,560 Yield per kw. h. do 2.80 Height of melt inches 46 Diameter of melt do 44 Diameter of electrode do 4 Rate of electrode movement inches/hour 4.50

EXAMPLE 3 The object of this example was to produce a composite product wherein crystals of one mineral substance were dispersed in a matrix of crystals of another mineral substance. A lithium hectorite-producing charge having the following ingredients:

Weight percent LiF 24.33 MgO 19.11 SiO 56.50

was blended, in a 1 to 1 weight ratio, with a fluorphlogopite-producing charge of a composition the same as that employed in the two preceding examples. Five hundred pounds of a 2,000 pounds batch of the total mixture was dumped into the bottom of the furnace. This initial charge was melted and the bottom portion thereof allowed to solidify before adding more feed material so as to form a micaceous insulation at the bottom of the furnace. Operation of the process was then resumed in the same manner as set forth in the preceding examples.

MELT STATISTICS Raw materials pounds 2,000 Total running time hours 9 Average power input kw 82 Peak power input kw 109 Total power input kw. h 728 Melt height inches 28 Melt diameter do 32 Crystalline mica product pounds 1600 Mica yield per kw. h do 2.2

The composite crystalline product at room temperature was placed in water. The water-swelling hectorite phase was then easily separated from the nonswelling, fluorphlogopite mica phase.

Among the many inorganic compounds and mineral substances that may be synthesized by the process of this invention are forsterite, magnesia, alumina and zirconia. As illustrated by Example 3, multicomposite crystal structures can be produced such as Ca SiO crystals dispersed in a crystalline matrix of spinel or magnesia, or dolomite dispersed in zircon. As long as the constituents are capable of being melted in the furnace, any multicomposite structure can be produced by the process of this invention.

High output of quality mica within a short period of time and lower power input per pound of product are the outstanding features of this invention. With the furnaces used, up to 7000 pounds of usable mica were obtained in hours. Total time involved is reduced by a factor of 2 to 4 in comparison with previous techniques. Furthermore, when the process is employed with substances of appreciable volatility, the maintenance of unmelted charge, consolidated and floating on the melt prevents the escape of volatile constituents.

While the particular process herein described is well adapted to carry out the objects of the present invention, it is to be understood that various modi-ficaitons and changes may be made all coming Within the scope of the following claims.

What is claimed is:

1. In a process for growing crystals of inorganic compounds or mineral substances along a thermal gradient in an electrode furnace wherein a small amount of furnace charge is initially melted and current passing between an electrode pair which is immersed in said melt causes a very large portion of unmelted charge to be readily melted, after which melted charge is cooled within said furnace to obtain a crystalline product, the improvement comprising melting said very large portion of unmelted charge by continuously maintaining relative vertical movement between said electrode pair and said furnace charge in such a manner so that said pair of electrodes are vertically displaced from their point of immersion in said initially melted charge along a substantially straight vertical path through unmelted charge at a rate suflicient to melt said very large portion and to establish a thermal gradient along said path, whereby crystals formed in said initiall melted charge will grow in the direction of warmer melted charge along said path of displacement of said electrodes; and maintaining a covering layer of unmelted charge on top of melted charge during said melting of said very large portion, said layer being sufficient to prevent escape of volatile constituents during said melting of said very large portion.

2. The process of claim 1 wherein said electrode furnace is an arc resistance furnace, and said small amount of furnace charge is initially melted by an are between the ends of said electrodes in close proximity to said small amount of furnace charge after which said ends of said electrodes are submerged in said initial melt.

3. The process of claim 2 wherein said electrodes are parallel to one another, and extend downward into said furnace through the top of said furnace and wherein said amount of charge is placed in the bottom portion of said furnace so that said electrodes initially melt said charge in said bottom portion and then are immersed therein after which said unmelted charge is placed on top of said initial melt, and wherein said relative movement between said electrodes and said furnace charge is maintained by continuously raising said electrodes through said unmelted charge on top of said initial melt.

4. The process of claim 3 wherein said unmelted charge is added to said furnace while said electrodes are being raised.

5. The process of claim 1 wherein said constant vertical movement is at a rate correlated with the optimum crystal growth rate of said crystals to obtain optimum length crystals for the ingredients of said furnace charge.

6. The process of claim 4 wherein said furnace charge is composed of ingredients that are capable of forming fiuormica crystals upon fusion and crystallization.

7. The process of claim 6 wherein said furnace charge consists of a mixture of potash feldspar, magnesia, alumina, silica and potassium silicofiuoride, and the product is fiuorphlogopite.

8. The process of claim 6 wherein said constant vertical movement is at a rate correlated with the optimum crystal growth rate of said crystals to obtain optimum length crystals for the ingredients of said furnace charge.

References Cited UNITED STATES PATENTS 2,923,754 2/1960 Worden 231l0 3,245,761 4/1966 Scott et a1. 23301 X 3,154,381 10/1964 Shell et al 23--110 OSCAR R. VERTIZ, Primary Examiner. A. GREIF, Assistant Examiner.

US. Cl. X.R. 23139, 301, 308. 

